1. Field of the Invention
This invention relates generally to apparatus and methods for performing measurements in a borehole. More particularly, the invention relates to sealing measurement instruments in a housing inserted in a borehole.
2. Description of the Related Art
In well logging, performed after a borehole has been drilled, and in measurement-while-drilling ("MWD"), performed during drilling of a borehole, measurement instruments are housed in a hermetically sealed housing, typically a metal cylinder, inserted down the borehole. The housing maintains a relatively lower internal pressure environment than that external to the housing and protects the instruments against fluid invasion and damage due to mechanical impact and compression forces within the borehole. The instruments are typically secured in a housing and an access port in the housing is sealed by bolting a cover plate over the port. It is well known to improve the sealing action of such cover plates with a resilient gasket between the cover plate and the housing. Also, a plug and resilient o-ring may be substituted for the cover plate and gasket. Certain problems exist with the above described sealing arrangement. Twisting and bending of the housing and cover plate or plug are particularly severe in MWD applications because of the forces associated with the vertical loading, friction, bending, and rotation of the drill string and the housing connected to the drill string. Moreover, this twisting and bending is repeated with each rotation of the drill string. Also, for MWD applications the housing is subjected to the flow of drilling mud at high pressures.
Because of the above factors, seal failure is a problem for these borehole related applications. For example, o-ring failure is commonly caused by a phenomenon which may be described as "dynamic extrusion failure." In this type of failure, the o-ring seals a gap between a plug and a port in the housing in which the plug is held. The gap repeatedly changes as the changing forces, such as bending and pressure, act on the housing, plug, o-ring and port. With the fluid pressure acting on the o-ring, the o-ring is repeatedly extruded into the gap as the gap increases with each cycle and is then pinched as the gap decreases. Over a period of many cycles an o-ring subjected to these conditions tends to fatigue and fail.